Systemic mastocytosis (SM), a myeloproliferative disorder with variable clinical manifestations, is associated in most cases with the D816V activating mutation in KIT which is the receptor for stem cell factor (SCF). Mastocytosis associated with germline KIT activating mutations is exceedingly rare and thus there are few studies on the effect of activating mutations on the biology of mast cells that continue to exhibit an activating mutation in KIT over time in culture. We were fortunate to be able culture mast cells from a patient who presented with unique clinicopathologic features of mastocytosis which was associated with a de novo germline KIT K509I mutation. To investigate the impact of the KIT K509I, primary human mast cells derived from CD34+ peripheral blood progenitors were examined for growth, development, survival and IgE-mediated activation. KIT K509I biopsied mast cells were round, CD25(-) and well differentiated. KIT K509I progenitors, cultured in SCF, demonstrated a ten-fold expansion compared to progenitors from healthy subjects. Further, they developed into mature, hypergranular mast cells with enhanced antigen-mediated degranulation. A KIT K509I mast cell transduction system revealed SCF-independent survival. Thus, this germline KIT mutation promoted a well-differentiated mast cell phenotype, distinct to that of somatic KIT D816V disease. As SM results from a mutation in c-kit in mast cells and we questioned if the function of bone marrow stromal cells (BMSCs; also known as mesenchymal stem cells) might be affected by the invasion of bone marrow by mutant mast cells. As expected, BMSCs from SM patients did not have a mutation in c-kit, but they proliferated poorly and osteogenic differentiation was deficient. Since the hematopoietic supportive abilities of BMSCs are also important, we studied the engraftment in NSG mice of human CD34(+) hematopoietic progenitors, after being co-cultured with BMSCs of healthy volunteers vs. BMSCs derived from patients with SM. BMSCs derived from the bone marrow of patients with SM were deficient in support of hematopoiesis. We also found significant differences between healthy and SM derived BMSCs in the expression of genes with a variety of functions, including the WNT signaling, ossification, and bone remodeling. These observations suggest that some findings associated with SM might be driven by epigenetic changes in BMSCs caused by dysfunctional mast cells in the bone marrow compartment. The diagnostic criteria for pediatric mastocytosis are largely based on adult studies and bone marrow findings are not well described in children. We thus evaluated use of the WHO criteria for diagnosis of systemic disease in pediatric mastocytosis. One hundred and thirteen children with pediatric mastocytosis were evaluated at the National Institutes of Health (NIH) between 1986 and 2013. Complete bone marrow evaluations were performed in 50 cases. Marrows were analyzed by histopathology, flow cytometry, and for KIT D816V. Bone marrow biopsies displayed mild atypical hematopoietic maturation. There was no evidence of peripheral blood cytopenias, myelodysplastic syndrome, myeloproliferative neoplasm or leukemia within this cohort. Based on these observations, we reached the conclusion that WHO criteria are applicable to the diagnosis of systemic mastocytosis in the pediatric population. Although unsuspected bone marrow findings typically seen in myeloproliferative disorders were observed, children within this study remained clinically stable without progression to a more aggressive variant. In a companion study, we examined clinical aspects of pediatric mastocytosis in relationship to serum tryptase and bone marrow pathology to provide practical guidance for management. We found that in children with high tryptase and severe mediator symptoms, all with organomegaly had systemic disease; none without organomegaly had systemic disease. Serum tryptase values differed significantly between urticaria pigmentosa and diffuse cutaneous and systemic mastocytosis and in all three categories versus controls; declined with time in most patients as did symptoms and was highly correlated with mast cells within the bone marrow of patients with systemic mastocytosis. There was a statistically significant relationship between clinical resolution and percentage decrease in tryptase. We concluded that children within all categories of disease either remained stable or improved. Organomegaly was a strong indicator of who needed a bone marrow biopsy. Serum tryptase, in addition to furthering classification, reflected bone marrow findings; while sequential tryptases were useful in supplementing clinical judgment as to disease course.